This invention pertains to a video signal encoding/decoding system, but more specifically, to a method and apparatus for restoring to proper video format a video signal which has previously been encoded with anti-copy protection signals. The invention is useful in restoring video signals which are prerecorded on a video recording medium, such as signals recorded on videotape in a video cassette.
Above-referenced U.S. Pat. No. 4,631,603 and copending patent application Ser. No. 935,055 disclose copy protection systems for use with video signals. The system disclosed in the '603 patent is finding significant usage with video signals on recorded video cassette tapes. The addition of the copy protection signal(s) to a prerecorded video cassette tape prevents unauthorized copying of the tape and still permits viewing of the program from the original tape. The copy protection systems include unique pseudo-synchronizing pulses and/or positive control signals placed at certain locations in the video signal having the effect of inhibiting proper AGC (automatic gain control) adjustment in a video tape recorder (VCR) during recording. As a result, picture quality of unauthorized copies is significantly deteriorated. However, these anti-copy signals do not affect normal viewing of the original tape.
To explain the copy-protect processes, a typical video signal defines normal viewing fields (e.g., those parts displayed on a television set) and vertical blanking intervals between the fields. Both the viewing fields and the vertical blanking intervals include series of horizontal scan lines carrying picture data and control information that includes for each line, a horizontal synchronizing pulse. The horizontal sync signals are used for horizontally registering successive scan lines. Other synchronizing pulses, e.g. broad pulses and equalizing pulses, normally occur during the vertical blanking interval. These pulses generally are called vertical synchronizing (or sync) pulses because they occur only during the vertical blanking interval. A television monitor or set, and also the processing circuitry associated with VCR recording, uses the vertical sync pulses for vertical roll synchronization and to trigger AGC gain adjustment. Alteration, modification or distortion of these signals can have a deleterious affect on picture quality.
Certain portions of a normal viewing field defined by a video signal typically are not used for defining the actual picture. More specifically, the horizontal scan lines immediately adjacent to those portions of a video signal defining vertical blanking intervals are not used in typical television monitors or sets to define the picture. Therefore those signal portions immediately adjacent to vertical blanking intervals are also available in their entirety for the addition of anti-copy protection signals. Thus, insofar as this invention is concerned, such signal portions are also be considered part of the vertical blanking intervals, and the term "vertical blanking interval" as used herein encompasses the same unless it is clear from the usage that only the actual vertical blanking interval is meant.
The aforementioned copy-protection systems change a normal video signal by adding positive (AGC) and/or pseudo-sync pulses after at least some of the normal sync pulses. By "added" pulses are meant pulses which are incorporated into the video signal to prevent copying, i.e., signals which supplement the normal video signal carrying typical picture data and control information. That is, the term "added" as used herein refers to the type of signal, rather than to the time at which it may be incorporated with the remainder of the video signal. In the arrangement disclosed in U.S. Pat. No. 4,631,603, the added pulses appear in the vertical blanking interval so as to interfere with a VCR's vertical synchronizing and/or AGC circuitry.
Under certain circumstances, there is a need to disable the copy protection system to permit recording of copy-protected recordings, such as, for example, during authorized and permitted copying or for studio editing purposes. Assignee's U.S. Pat. No. 4,695,901 discloses several embodiments of one such system. It is an objective of the present invention to provide an improvement for disabling an anti-copy system.